Structural Factors Determining the Thermal Decomposition Temperature of Phosphonium-Type Ionic Liquids On Metal-Oxide Supports

Structural Factors Determining the Thermal Decomposition Temperature of Phosphonium-type Ionic Liquids on Metal-oxide Supports

Volkan Balci, Asli Akcay, Alper Uzun*

Department of Chemical and Biological Engineering, Koç University, 34450 Sarýyer, Istanbul, Turkey
*Corresponding Author: auzun@ku.edu.tr

Ionic liquids (ILs) are new-generation green solvents and potential alternatives for their volatile organic counterparts. They have received tremendous attention during the last decade because of their unique physical and chemical properties such as non-volatility, non-flammability, high chemical and thermal stability, high solvating ability and tunable miscibility. These physicochemical properties can be tailored by incorporating almost infinite combinations of different anions and cations; hence they are often called as “designer” or “task-specific” solvents. Such structural diversity and functional flexibility has led to a growing interest towards application in numerous research fields such as synthesis, electrochemistry, separation, and catalysis.

As one of the emergent application areas, two primary concepts have been pioneered to exploit tunable features of ILs in catalysis -supported ionic liquid phase (SILP) and solid catalysts with an ionic liquid layer (SCILL). For SILP-type catalyst, a thin layer of ionic liquid containing a homogeneous catalyst is applied to the internal surface of a porous support material. Similarly, in SCILL concept, heterogeneous catalyst or catalytically active materials are coated with a thin film of IL. These promising catalytic concepts lead to enhanced selectivity, product distribution, yields, and selective solubility for intermediates and products because of promoting interactions between active sites, supports and IL.  However, application of ILs in these supported-catalyst concepts is limited with their thermal stability on corresponding support materials. Though most of the ILs are thermally stable at elevated temperatures (> 300 ^°C) in their pure state, they become less stable when coated on metal-oxide supports because of the interactions between IL and metal-oxide.  Here we report the results of a systematic investigation on the elucidation of structural factors determining the thermal stability limits of 12 different phosphonium type ILs on three of the most commonly used catalyst supports, SiO_2, g-Al₂O₃, and MgO.

Thermogravimetric analysis (TGA) and IR spectroscopy based results illustrate that thermal stability of phosphonium type ILs on metal-oxide supports mainly depends on the point of zero charges of metal-oxides and the structure of ILs.  In general, the thermal decomposition temperature of phosphonium-type ILs decreases significantly as the acidity level of support decreases from SiO₂ to g-Al₂O₃ and to MgO.  For instance, derivative onset of the decomposition temperatures for tetrabutylphosphonium methanesulfonate decreases from 372 °C to 212 °C as the metal-oxide changes from SiO₂ to MgO.  

Moreover, data further illustrate that the thermal decomposition temperature is strongly influenced by the physicochemical properties of ILs determined by their structure.  Figure 1 summarizes these variations in thermal decomposition temperature with varying phosphonium-type IL structure on different metal-oxide surfaces. Results show that thermal stability of ILs on the same metal-oxide decreases significantly with increasing anion size. For example, for trihexyltetradecylphosphonium cation, the thermal stability of ionic liquid with bis(2,4,4-trimethylpentyl)phosphinate anion decreases compared to those with smaller anions such as bromide.  When compared with these halide anions, a switch from Br^- to more electronegative Cl^- anion further decreases the decomposition temperature by approximately 20 °C on all supports.  On the other hand, thermal stability of ILs on metal oxide supports increases when an aromatic group is inserted into a methansulfonate group. For instance, on g-Al₂O_3, thermal stability increases from 259 °C to 291 °C when the methanesulfonate anion is substituted with p-toluenesulfonate.

Results presented here reveal the structural factors determining the thermal stability limits of phosphonium-type ILs on three of the most commonly used metal-oxide catalyst supports. These stability limits serve as the basis for the selection of suitable phosphonium-type ILs according to the application conditions of IL-assisted supported catalysts. Thus, they add to the gap in IL literature focusing on the properties of ILs on metal-oxide supports.